Swift class conforming to Objective-C protocol

Question

In Objective-C I have the the following protocol :

@protocol GCKDeviceScannerListener <NSObject>    
@optional

- (void)deviceDidComeOnline:(GCKDevice *)device;
- (void)deviceDidGoOffline:(GCKDevice *)device;
- (void)deviceDidChange:(GCKDevice *)device;

@end

When trying to conform to this protocol in Swift Xcode 6.1 autocompletes it like this:

class ViewController: UIViewController, GCKDeviceScannerListener {

    override func viewDidLoad() {
        super.viewDidLoad()
        var deviceScanner = GCKDeviceScanner();
        deviceScanner.addListener(self);
        deviceScanner.startScan();
        println("scanning");
    }

    func deviceDidComeOnline(device: GCKDevice!) {
        println("deviceDidComeOnline()");
    }

    func deviceDidGoOffline(device: GCKDevice!) {
        println("deviceDidGoOffline()");
    }

    func deviceDidChange(device: GCKDevice!) {
        println("deviceDidChange()");
    }

}

The code compiles and seemingly runs ok on the simulator. However, none of the listener functions are ever triggered. Everything works 100% of the time when running the demo project from Google written in Objective-C only. Because of the last part I'm assuming that the there aren't any problems with the network or hardware or anything like that.

It could be that I have missed something important from https://developers.google.com/cast/docs/ios_sender , but I would like to know if the Swift code itself is correct according to the protocol . As the protocol only has optional functions it's hard to know if it's right.

Answer 1

I have no experience with this library, but I think you should keep the reference to GCKDeviceScanner .

Try:

class ViewController: UIViewController, GCKDeviceScannerListener {

    var deviceScanner = GCKDeviceScanner()

    override func viewDidLoad() {
        super.viewDidLoad()
        deviceScanner.addListener(self)
        deviceScanner.startScan()
        println("scanning")
    }

Answer 2

Apple's documentation on Protocols is long and complex.

It's easiest to think of optional protocol methods like Optional closures, and you can use it with optional chaining.

@objc class Something {
    var delegate: GCKDeviceScannerListener?

    func someCallback() {
        delegate?.deviceDidComeOnline?(device)
    }
}